Vapor compression systems (VCSs) are the most widely used cooling systems for refrigeration, air conditioning, and heat pumps. VCSs consume significant electrical energy and their potential environmental impact has prompted development of more eco-friendly alternative technologies. In contrast to VCSs, absorption refrigeration systems (ARSs) utilize low-grade thermal energy and use eco-friendly refrigerants, such as water. ARSs are often used in large-scale applications where a thermal energy source or excess heat from a process is available. Presently, ARSs are not economically competitive with the VCSs in small-scale applications due to their high initial cost. If high performance, inexpensive, and robust ARSs could be developed, they could play a significant role in the future energy economy.
One of the main components of an ARS that has a significant impact on its size, cost, and performance is the absorber heat exchanger. In an absorber, the refrigerant molecules are absorbed into an absorbent by an exothermic condensation from vapor to liquid. Additional heat is generated due to interactions between the refrigerant and absorbent molecules. The generated heat must be removed from the absorbent to perpetuate the absorption process. In existing systems, a falling film is utilized to generate a significant vapor-absorbent interface and to facilitate heat removal from the absorbent as it falls on a cold surface. Since heat is generated at the vapor-solution interface and must transfer through the solution film to reach the cold surface, the liquid film thickness plays a major role in the absorption process. A thick solution film displays significant mass transfer resistance as the refrigerant molecules diffuse through the solution.
Enhancement of the absorption rate and development of scalable absorber configurations have been intensively studied. Falling film absorption processes over vertical walls, horizontal and vertical tube banks, and helical coiled tube configurations have been numerically and experimentally examined. Falling films over a horizontal tube bank is the arrangement commonly implemented in existing large-scale systems. To develop compact absorbers, alternative configurations have been explored. Most recently, the efficacy of the membrane-based absorption process and its scalability have been demonstrated. Nasr et al., “Absorption characteristics of lithium bromide (LiBr) solution constrained by superhydrophobic nanofibrous structures”, Int J Heat Mass Transf, 2013; 63, 82-90 reports absorption rates 2.5 times higher than that of the conventional falling film absorbers. Bigham et al., “Moving beyond the limits of mass transport in liquid absorbent microfilms through the implementation of surface-induced vortices,” Energy 2014; 65, 621-30, numerically shows enhancement in the membrane-based absorption process can be achieved through generation of vortices within the flow through implementation of micro-scale features on the flow channel wall. The vortices change the mass transfer mode within the solution from diffusive to advective transfer. Nevertheless, non-membrane based absorbers that have high efficiencies remain a goal for ARSs, particularly absorber designs suitable for the plate-and-frame absorber configurations.